1. Field of Invention
This invention relates to a method of processing a workpiece in a chamber.
2. Background of Invention
There are nowadays many manufacturing techniques in which workpieces are treated using a first gas or gas combination, in which at least one of the gases is reactive, and then the workpiece is subjected to a second process. In many cases, both for chemical and processing reasons and for workpiece throughput reasons, it is desirable to perform the second process in the same chamber as the first, often without the chamber being exposed to atmosphere. In such circumstances a reactive gas can become absorbed on or linger in those parts of the gas delivery system and can become active during the second process, particularly if it is an energetic process, such as a plasma process.
In those circumstances if, for example, the first process is a deposition process, then additional deposition may take place during the second process resulting in over thick films and loss of uniformity. One example of such a multi-step process is the deposition of thin layers, e.g. TiN using a metal organic chemical vapour deposition (MOCVD) vapour such as Tetrakis (Diethylamido) Titanium (TDEAT) and NH3 as precursors. Such a process is described in detail in U.S. Pat. No. 5,139,825, which incorporated herein by reference.
The main application of such TiN films is to coat high aspect ratio feature devices, such as trenches and contact structures on wafers containing microelectronic devices. For example TiN is used as a liner between Ti and WF6 during W plug formation. The TiN liner prevents an adverse reaction that occurs between Ti and WF6, thus allowing formation of a pure W plug in the contact.
In order to prevent any intermixing of the reactants prior to entering the chamber, and hence preventing the unwanted deposition within the gas supply lines, it is typical to have the reactive gases entering from different inlets, such as via a dual port showerhead.
It is important that there is a continuous coating of TiN and preferably this coating should be as conformal as possible. The coatings are typically less than 15 nm in thickness with some manufactures using as little as 5 nm. This thickness is likely to reduce even further as microchip critical dimensions decrease. Such thin coatings require very little process gas.
It has been found that TiN deposited using these or similar precursors are prone to absorbing oxygen and moisture on exposure to atmospheres and that a subsequent in situ H2/N2 plasma treatment of the film reduces oxygen uptake, increases density and makes the fill resistivity more stable and improves the line of properties of the TiN. The plasma treatment has to be performed without a wafer breaking vacuum and this is an example of a case where it is advantageous to be able to deposit a plasma treat in the same process chamber.
In unpublished work the Applicants have determined experimentally that TiN deposition can occur during the subsequent in situ H2/N2 plasma step thus increasing the film thickness. This is reported more fully below.
Current teaching would suggest that in order to overcome the problem that the Applicants have identified, one should pump or gas purge the TDEAT line after deposition, for example using the kind of arrangement described in U.S. Pat. No. 5,772,771. However, the Applicants have determined, in unpublished work, that TDEAT confirmly decompose in an uncontrolled manner on the wafer surface during purging and that purging takes in excess of several minutes which will impact wafer throughput.